Direct current—DC—offset is a distortion that might lead to a performance decrease at an output of an analog-to-digital converter of any receiver. DC offset can be defined as a deviation from zero of the mean amplitude of the analog signal to be converted to a digital signal.
DC offset is frequently encountered in radio frequency—RF—receivers while generating a baseband signal, e.g. in so-called direct-conversion receivers that are used In telecommunications to demodulate incoming RF by mixing it with a local oscillator signal synchronized in frequency to the carrier of the wanted signal.
FIG. 1 shows a basic structure of a direct conversion receiver according to the prior art, direct conversion receiver by way of example comprising a bandpass filter or diplexer 102, a pre-amplifier and down conversion circuit 104, this circuit comprising a first variable gain amplifier 1042, a local oscillator 1044, and a mixer or down converter 1046, a second variable gain amplifier 106, an analog-to-digital converter 108 and baseband processor 110 connected in series. A RF signal S_RF is received by an antenna and provided to the bandpass filter 102 that generates a bandpass signal S_BP by selecting one carrier frequency out of a plurality of carrier frequencies of the RF signal S_RF. The first variable gain amplifier 1042 generates an amplified bandpass signal from the bandpass signal S_BP according to a first gain control value Gain—1. The Mixer 1046 receives both the amplified bandpass signal and an oscillator signal generated by the local oscillator 1044 and generates a down-mixed signal S_BB, in the following also being referred to as baseband signal S_BB. The second variable gain amplifier 106 generates an amplified down-mixed or analog signal S_A from the down-mixed signal S_BB according to a second gain control value Gain—2. The analog-to-digital converter—ADC—108 converts the analog signal S_A into a digital signal S_D that is provided to the baseband processor 110 for further digital processing.
In order to achieve a trade-off between distortions caused by signal clipping or saturation and distortions caused by signal quantization at the ADC 108, the total gain Gain—1×Gain—2 might be adjusted such that a so-called target value for the power of the baseband signal after analog-to-digital conversion—ADC—is obtained, e.g.
      20    ⁢          log      10        ⁢          {                                                  γ              ADC                        ·            Gain_                    ⁢                      1            ·            Gain_                    ⁢                      2            ·                                          P                RF                                                    FS            }        =      Target    ⁢                  ⁢          Value      ⁢                          [      dBFS      ]      wherein FS corresponds to the full scale value per quadrature component after ADC, PRF is the power of the RF signal at the antenna within the system bandwidth considered, and
      γ    ADC    =      σ          P      is the nominal gain due to the ADC, whereby σ is the standard deviation of the complex valued baseband signal per quadrature component after ADC, and P is the power of the baseband signal before ADC. It might be noted that γADC itself is also dependent on the target value in dBFS since clipping or saturation of the signal at the FS value reduces the average power σ2 of the signal.
One reason for DC offset within direct conversion receivers is local-oscillator energy leaking through the mixer to the antenna input and other parts of the receiver front-end circuitry and then re-entering the mixer (the effect is also being referred to as self-mixing). The DC offset might additionally depend on specific implementation details, e.g. crosstalk attenuation, shielding and element tolerances. The DC offset however causes a degradation of the functioning of the receiver, e.g. due to saturation or clipping effects if not being accounted for.
It is known to remove a DC offset by filtering e.g. with a DC notch filter. DC notch filtering can significantly attenuate or almost completely remove the DC offset, but it might introduce significant distortions to the frequency characteristics (e.g. subcarriers) near to DC. In order to minimize such distortion, a DC notch filter would need to have a very narrow lowpass filter characteristics, which is difficult to realize. However, practically used DC notch filters have a relatively smooth transition band between the passband at the low frequencies and the stopband at the higher frequencies. Such filter also attenuates at frequencies near to DC. Further, a narrow lowpass characteristic in the frequency domain corresponds to a long impulse response in the time domain, which might cause inter-symbol interference when the composite channel impulse response exceeds a certain length. Moreover, practical DC notch filter implementations often introduce group delay variations around the DC subcarrier, which causes larger channel estimation losses in the baseband processing.